Rare earth catalyst systems, such as lanthanide systems, are normally considered to be "pseudo-living." The polymers made utilizing such rare earth catalyst systems accordingly increase in molecular weight with increasing monomer conversions. The molecular weight of polymers made utilizing such catalyst systems have traditionally been regulated by limiting conversions and/or adjusting the level of catalyst utilized. In any case, any change in polymerization time or conversion has traditionally affected the ultimate molecular weight of the polymer being synthesized. Since residence times in continuous polymerization systems are sometimes difficult to control precisely, linear relationships between molecular weight and monomer conversion causes the production of polymers having varying molecular weights. In other words, the molecular weight of polymers made utilizing rare earth catalyst systems is often difficult to control.
In polymerizations which are conducted on a commercial basis, it is often highly advantageous to attain high monomer conversions. Attaining high monomer conversions is particularly beneficial in bulk polymerizations. However, there is a demand for polymers made using rare earth catalyst systems which have low or only moderately high molecular weights. Unfortunately, traditional molecular weight modifiers which are effective for use with Ziegler-Natta catalyst systems are ineffective for use in polymerizations which are catalyzed with rare earth systems. For instance, .alpha.-olefins and nonconjugated diolefins are effective chain transfer agents for use with Ziegler-Natta catalyst systems (see U.S. Pat. No. 4,383,097 and South African Patent No. 83/2555). However, .alpha.-olefins and nonconjugated diolefins are ineffective as molecular weight regulators for use with rare earth catalyst systems. The molecular weight of polymers made with Alfin catalysts can be reduced by conducting such polymerizations in the presence of 1,4-and 1,2-dihydronaphthalene. It has, however, been determined that 1,4- and 1,2-dihydronaphthalene cannot be used as molecular weight modifiers in polymerizations which are catalyzed with rare earth metal catalysts.
There are many benefits which are derived from using preformed rare earth metal catalyst systems. Unfortunately, the problem of molecular weight control may be further complicated by utilizing preformed catalyst systems since they produce polymers which have higher molecular weights than those made with catalysts which were not preformed.